The invention relates to a drying apparatus for the drying of bulk material, more particularly synthetic resin granules or powder.
It is more especially in the synthetic resin processing industry that it is frequently necessary for raw materials in the form of granules, powder or in a similar bulk condition to be dried, before same are for example processed in an injection molding machine or in an extruder. For this purpose hot gas driers are known, in the case of which the gas employed for drying is heated prior to the drying operation to a temperature specific to the synthetic resin and the hot gas is then passed through the material to be dried.
In order to ensure effective drying of the material to be dried on the one hand while at the same time avoiding unnecessary waste of energy on the other hand it is necessary to hold the drying gas flowing through the material at a predetermined temperature within the drying container. For this purpose switching and regulating means are known, which find the temperature at the outlet of the drying container by means of a temperature sensor, which is connected by means of a capillary tube connection with a thermostat (or flow rate control valve) arranged in the gas supply duct. In the event of the temperature increasing at the outlet of the drying container, for example because a trouble condition has occurred, and the heated synthetic resin material is no longer removed from the drying container, the thermostat reduces the passage of gas to the drying container. If the gas outlet temperature falls below a predetermined value, the gas volume flow rate to the drying container will be increased.
It is however a disadvantage in this respect that thermostat-operated valves capable of operating precisely are extremely expensive and consequently do not come into question for small drying plant. In the case of known drying plant use is consequently usually made of cheap but extremely inaccurate thermostat-driven valves for volume flow rate regulation of the drying gas. A further disadvantage is that thermostat-operated regulation valves, which are connected via a capillary tube connection with the temperature sensor, respond with a long time lag to changes in temperature. A rapid correction of the gas volume flow rate is therefore not possible. More particularly in the case of small drying gas volume flow rates of below 1000 1/min the known thermostat regulation means react slowly and thus lead to inaccurate process control. A still further disadvantage is that using conventional thermostat regulation means it is not possible to select any drying gas flows specific to the material to be dried, i.e. to use different drying programs, as they are for example necessary of different types of synthetic resin, such as ABS or PET.